Marijuana has grown more potent over the decades. A joint now contains about three times the tar of a cigarette, plus known
cancer-causing carcinogens such as vinyl chloride, dimethynitrosamine, methylethynitrosamine, benz(a)anthracene, benzo(a)pyrene,
as well as carbon dioxide, ammonia and more than 20 other major toxins. Because users inhale longer, five joints a week have
the impact of a pack of cigarettes a day.

(SOURCE: UNIVERSITY OF CALIFORNIA AT LOS ANGELES AND NEW ENGLAND JOURNAL OF MEDICINE)

While it may be true that sinsemilla is more widely available than 10 or 15 years ago, its potency has not changed significantly from the 2.4 to 9.5 percent THC materials available in 1973-1974 (see Table I), or the five to 14 percent sinsemilla of 1975 (Perry 1977). The range of potencies available then (marijuana at 0.1% to 7.8% THC, averaging 2.0% to 5.0% THC by 1975) was approximately the same as that reported now. With such a range, the evidence simply cannot support the argument by Cohen (1986) that marijuana is "ten or more times more potent than the product smoked ten years ago." And to say that marijuana potency has increased 1,400 percent since any date in history is patent nonsense.

The claim that there has been a 10-, 20- or 30-fold increase in marijuana potency since the 1970s is used to discredit previous studies that showed minimal harm caused by the drug and convince users from earlier eras that today's marijuana is much more dangerous.

THE FACTS

For more than 20 years the government-funded Potency Monitoring Project (PMP) at the University of Mississippi has been analyzing samples of marijuana submitted by U.S. law enforcement officials. At no time have police seizures reflected the marijuana generally available to users around the country and, in the 1970s, they were over- represented by large-volume low-potency Mexican kilobricks. 8

During the 1970s, the PMP regularly reported potency averages of under 1%, with a low of 0.4% in 1974. Quite clearly, these averages underestimate the THC content of marijuana smoked during this period.
Marijuana of under 0.5% potency has almost no psychoactivity. While it is possible that people sometimes obtained marijuana of such low potency, for the drug to have become popular in the 1960s and 1970s, most people must have regularly obtained marijuana with higher THC content.

Until the late 1970s, PMP samples included none of the traditionally higher-potency cannabis products, such as buds and sinsemilla, even though these products were available on the retail market. When changes in police practices resulted in their seizure, PMP potency averages increased.

Every independent analysis of potency in the 1970s found higher THC averages than the PMP For example, the 59 samples submitted to PharmChem Laboratories in 1973 averaged 1.62%; only 16 (27%) contained less than 1% THC, more than half were over 2% and about one-fifth were over 4%. In 1975, PharmChem samples anged from 2 to 5%, with some as high as 14% - nearly 30 times the .71 average reported by the PMP 9
After 1980, both the number and variety of official seizures increased dramatically, improving the validity of the PMP's reported averages, although they continue to be based on "convenience" rather than "representative" samples.

As shown below, average potency has remained essentially unchanged since the early 1980s:

Even if potency had increased slightly since the 1970s, it would not mean that smoking marijuana had become more dangerous. In fact, since the primary health risk of marijuana comes from smoking, higher potency products can be less dangerous because they allow people to achieve the desired effect by inhaling less.